Creating Waves

The redistribution of heat between the equator and poles drives a series of atmospheric circulations that cause air to rise and fall at different latitudes around the planet. Combined with the Coriolis forces from the spin of the earth, this air movement generates strong westerly winds at ±40° ("the roaring forties") and easterly winds around the equator ("trade winds").

As these winds blow across a body of water, energy is transferred from the air to the water, and waves are created. The increased roughness of the water surface caused by these waves improves the transfer of energy from the wind to the waves, which in turn makes the waves larger and the surface rougher.

Thus strong winds and/or long stretches of water both lead to large waves with more energy.

Global wind patterns
igure 7.5 in The Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001.
See also Atmospheric Circulations

Absorbing Energy

Consider for a moment the impact of waves on a fixed vertical wall, which reflects all the incident wave energy. In this instance, a standing wave pattern forms from the constructive interference of the incident and reflected waves that carry energy in opposing directions.

Conversely, to effectively absorb all the incident wave energy, the wall needs to move so that it creates a wave exactly out of phase with the incident wave. If these waves perfectly cancel each other out no energy remains in the water at the wall's boundaries, and all of the original energy must have transferred to the moving wall - thus we've created a wave energy absorber!

However, this creates two challenges:

  • any movement that is in excess of what is required along that boundary will dissipate energy to the surrounding water; and
  • insufficient movement along any boundary will reflect useful energy.

Therefore an ideal wave energy device should be shaped to minimise energy dissipation and reflections and its motion should be suitably damped for a given wave condition. However, before we seek to solve this seemingly simple problem, we must also remember that real ocean waves are not regular: they vary in intensity, direction, aren't sinusoidal and the energy is dispersed throughout the water column!

Unphased wave entering on a flat surface
Standing wave pattern from constructive interference.
Phased wave entering on a flat surface
Incident and reflected wave perfectly out of phase.

Ocean Wave Energy

It is important to remember that waves exist because of a movement of energy through the water. This wave energy has two key components: Potential Energy due to the water surface elevation and Kinetic Energy due to the movement of the water particles.

Much like a swinging pendulum, the amount of potential and kinematic energy varies throughout the wave cycle. Larger waves occur where there is a concentration of energy, whereas flatter (or "calmer") water represents an absence of wave energy.

The available energy at any one point can be heading in many different directions, depending on its origin:

  • Local Waves - are created by nearby wind conditions and are characterised by a relatively high frequency and steepness. They tend to follow the prevailing wind.
  • Swell Waves - are formed by gales far away and have evolved with longer wavelengths that can efficiently travel large distances. These waves are easily diffracted and reflected by land-masses, with the direction of this energy mainly determined by the origin: for much of the UK and Europe swell waves arise from storms across the mid-Atlantic.
Frequency Spectrum
Empirical wave spectra for fully developed seas at different wind speeds.
Directional Spectrum
Directional distribution of waves.